There are approximately 100,000 individuals with an arm absence or complete hand absence in the United States, and perhaps as many as 10,000 new cases each year. Electric hands are becoming more widely adopted as the commercially available components have become more dependable and the function of electric prostheses has improved. Some market studies indicate that, in the clinics specializing in upper limb prosthetics, electric hands may be used in at least one-half of the new prosthetic fittings. Contributing to this trend, training for prosthetists in fitting skills and techniques is part of the curriculum of accredited programs and is widely provided by manufacturers.
Prosthetic hands and arms that are powered can be controlled by using myoelectric control. Myoelectric control is an effective, comfortable and natural way to control artificial arms and hands. Small, stainless-steel electrodes are placed on the skin and sense the electrical activity of the muscles using the electromyographic signals. These muscle signals are electronically amplified and used to move the hand as desired.
The electrodes are installed in the prosthesis socket when the arm is fitted, then connected to the electronics of the prosthetic arm and/or hand system. Retraining of the patient's muscles may also be needed to build strength and control. Some types of myoelectric systems use proportional control, where the amputee can move the hand slowly or quickly, instead of simply turning it on and off. A high sensitivity in the prosthetic system provides a more natural response with less effort.
Current electric hands or terminal devices (TDs) do not generally provide the wearer any direct feedback or sensation about the pinch force generated by the hand. Individuals who have not lost a hand or limb do not realize the importance of being able to sense how hard or soft an item is being handled. A natural hand provides detailed feedback using an extensive nerve network in the skin and other related information can be provided using the muscles. Without this feedback, it can be easy to damage objects that are handled with a prosthetic hand.
Body-powered TDs can give the wearer a very limited amount force sensation and proprioception through the cable force and position. Unfortunately, harnessed feedback uses more harnessing and a control cable, which in turn causes discomfort to the prosthesis wearer.
In comparison, an electric prosthesis does not provide feedback regarding the powered grip or movement of the prosthetic. With an electric hand, the wearer's control over pinch force is only approximate and depends heavily upon visual feedback. Using visual feedback is not easy when an object is hidden from view inside the hand or is viewed in dim light. In addition, visual feedback is not generally helpful when the resistance of a hard or soft object is not known in advance. In other words, soft objects can sometimes appear hard but then when force is applied the prosthesis user may quickly find out visually that the soft object has been damaged.